The present invention relates to a load sensing variable displacement hydraulic motor which adjusts displacement in response to the load on its output shaft. The invention relates particularly to a hydraulic motor which operates in a low displacement, high speed mode when the torque load on its output shaft is low. The motor's displacement increases when the torque load on the output shaft increases.
In the prior art, there are disclosures of hydraulic motors whose displacement can be changed by an operator to change the speed and the output torque of the motor. For example, in U.S. Pat. No. 3,687,578, a hydraulic motor has rotatable gears with intermeshing teeth defining fluid chambers that may be expanded and contracted in order to rotate the gears and thereby drive an output shaft coupled to one of the gears. In the patent, one gear is shifted relative to another gear (either axially or angularly) to change the volume of the chambers, and thereby change the hydraulic displacement of the motor. In U.S. Pat. No. 3,687,578, the displacement of the motor is changed by external means (e.g., an external source of fluid, a hand crank) which shifts the gears relative to each other in order to change the volume of fluid displaced by the motor.
Another example of a hydraulic motor with a variable volume displacement is shown in U.S. Pat. No. 3,200,756. That patent discloses a vane-type motor with a cam ring which is biased to a position in which its inlet opening has a maximum flow area to provide a maximum displacement. In response to an increasing speed on its output shaft, centrifugal force increases the frictional engagement between the vanes and the cam ring. The cam ring then rotates to decrease the size of the inlet opening, to reduce the displacement of the motor. Thus, the motor of the patent is biased to its high displacement, high torque, low speed mode, and responds to increasing shaft speed to decrease its displacement and its output torque.